The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
The field of this invention is that of prevention and/or removal of blockages in pipelines, such as remote subsea pipelines. Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40° F. or less.
The hydrocarbon fluids thus recovered, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable.
Similarly, hydrocarbon gas under pressure combine with water at reduced temperatures to form a solid material, called a ‘hydrate’. Hydrates are generally a porous solid which is formed primarily of water with a mixture of gases. It is effectively similar to ice. There is a tendency for hydrates to form in the pipelines departing from a subsea gas well, especially on well start-up. The temperature of seawater at depths will often approach 32° F., with the temperature in non-flowing pipelines being the same. When a subsea pipeline valve is opened, the gas expansion can cause substantial additional cooling. In these cold and high pressure conditions, hydrates of the gas and water can quickly form.
Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressurizing the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create more operational problems and be costly in terms of lost production. For example, frequently when hydrates form, it forms a blockage that will be somewhat porous. At that time, a high pressure will exist on the upstream side and a lower pressure will exist on the downstream side of the blockage. This means that additional gas will move thru the hydrate plug and expand and therefore cool as it does. This means that not only can the expansion of this gas keep the formed hydrates cool, but can literally continue to grow additional hydrate blockage.
Similarly, paraffins can form blockages in pipelines by building up on the inner diameter of the cold pipelines as relatively warm hydrocarbon fluid circulates out of a well and cools as it flows down a subsea pipeline. As the layer of paraffin builds up on the subsea pipeline inner diameter, the inner diameter of the paraffin becomes smaller and smaller. Ultimately a pigging device intended to clean the paraffin will cause the paraffin to separate from the inner wall of the pipeline and become a plug. In some cases the paraffin will release from the subsea pipeline inner diameter without a pig and cause a blockage. In either case, if the pressure in the pipeline is enough to move the plug along the pipeline, it will continue to collect additional paraffin as it moves until the length of the blockage cannot be moved by the available pressure.
The problem of lower temperatures in subsea pipelines has been addressed by placing thermal insulation on the lines, but the length of some pipelines makes thermal insulation alone ineffective. Increased flow rate through the lines also helps to minimize temperature loss of the fluids, but flow rate varies and is determined by other factors. Problems of heat loss from a pipeline increase late in the life of a hydrocarbon reservoir because production rates often decline at that time. Problems become particularly acute when a pipeline must be shut-in for an extended period of time. This may occur, for example, because of work on the wells or on facilities receiving fluids from the pipeline or hurricane shutdown. The cost of thermal insulation alone to prevent excessive cooling of the lines becomes prohibitive under these conditions.
Conventional solutions include heating of pipelines by bundling the lines with a separate pipeline that can be heated by circulation of hot fluids. Other prior art include devices as found in U.S. Pat. No. 6,939,082 describing a method of taking a remotely operated vehicle (ROV) to the ocean floor to land on and move along a subsea pipeline above or below the seafloor and repeatedly circulate seawater which has been heated electrically, mechanically, or chemically across the outer surface of the pipeline to melt hydrates or paraffins which have formed on the inside of the pipeline.
Yet other attempts have been made to enter the end of the pipeline with a somewhat flexible string of coiled tubing to get to the blockage and wash it out. This is an expensive operation, and in some cases the blockage can be 10 to 20 miles away. Removal by use of coiled tubing can be further complicated if the pipeline has bends in it, making passage of the coiled tubing difficult if not impossible.
Still other proposed solutions involve human divers servicing subsea pipelines, or via a technique called ‘hot tapping’, which involves connecting to an existing pipeline without interrupting pipeline flow.
All the conventional methods are generally costly, dangerous and with unpredictable results, even in situations where such methods are practical. The present invention seeks to propose possible solutions, at least in part, in amelioration of the shortcomings in the art.